Reconstructing the Principled Shader SSS with standard nodes
Unfortunately, the Principled BSDF node does not use the same subsurface scattering algorithm as the algorithms accessible through Cycles nodes. The paper  that the Principled algorithm comes from states:


  Our subsurface parameter blends between the base diffuse shape and one inspired by the Hanrahan- Krueger subsurface BRDF. This is useful for giving a subsurface appearance on distant objects and on objects where the average scattering path length is small; it’s not however a substitute for doing full subsurface transport as it won’t bleed light into the shadows or through the surface.


Cycles supports Christensen-Burley, Random Walk, Cubic, and Gaussian algorithms. Hanrahan-Krueger is not one of them. Additionally, the paper states that the algorithm is "inspired by" the Hanrahan-Krueger algorithm, so it doesn't even utilize it exactly.

Looking at the above quote, though, it appears that the Principled subsurface algorithm is not as accurate as those within Cycles, so it may suit your purposes better to just use one of algorithms already included in the Subsurface Scattering node1. 

If you really need to use the subsurface algorithm in the Principled shader, you can find the .brdf code here on GitHub. The relevant code begins on line 110. You can convert this to OSL code and then add it into the NodeTree with a Script node. 



1 Interestingly, the author of the paper that details the Principled algorithm is Brent Burley, namesake of the more accurate Christensen-Burley algorithm included in Cycles.